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pH Measurement in Industrial Waters

By Chemical Engineering |

Determination of pH is a critical and ubiquitous operation whenever water is used in the chemical process industries (CPI). This one-page reference provides a review of pH chemistry and information on pH measurement equipment in CPI applications. Chemistry of pH The quantity pH is a measure of the acidity or basicity of an aqueous solution. It is expressed as the negative logarithm of the activity of hydrogen ions in solution (pH = – log aH +; where aH + is the activity of hydrogen ions). Effectively, the activity represents the concentration of hydrogen ions. However, hydrogen ions (that is, free protons) do not exist by themselves in solution. H + ions quickly bond with surrounding water molecules, making H 3 O + (hydronium ion). Because the pH scale is logarithmic, a change of one unit of pH corresponds to a ten-fold change in hydrogen ion concentration. In a sample of pure water, it has been determined experimentally that the auto-ionization of water results in a H + concentration of 10 –7 mol/L, which is equal to the concentration of OH – ions. Because of this, neutral pH is said to be equal to 7.0. Values below 7 are acidic, while those above 7 are basic. pH electrodes Measurement of pH is generally accomplished with…
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